I mean, you're correct that Obsidian doesn't run on the browser. But it's built on web technologies. As a result, I would argue that the overlap between the skillset and work needed to build an app like Obsidian overlaps more with most web applications than most desktop and mobile applications.
You're also correct that Electron provides APIs beyond those available in the browser, such as access to the native filesystem. The way I see it, those are mostly some lower-level details it wouldn't be that hard to run Obsidian in the browser, it's just a product choice not to (specifically, it would imply creating a file hosting service). As the Obsidian mobile app demonstrates, Electron is swappable and not needed for Obsidian's core functionality. In contrast, had Obsidian been built on React, it would be rather difficult to simply "run without React" without rewriting the entire application.
How to build a large front-end app on non-web technologies (Swift, C++/QT, C#, etc) is also an interesting question but I didn't understand that as being the topic of this conversation.